Human accessible areas of electronic circuitry are subject to stringent safety requirements and regulations. One way to meet these safety regulations is for the human-accessible areas to be constructed as Safety Extra Low Voltage (SELV) areas. As defined under the current IEC regulations, these SELV circuits would be expected to be safe under both normal and single fault conditions.
On some occasions, the electrical power supplies for the SELV area may be supplied via a high voltage area, for example, from the mains electrical power supply. Therefore, it is desirable to be able to transfer power from this unsafe, high voltage area to the SELV area. This may be done by providing circuitry from the mains to the SELV area which is fault tolerant, in order to provide safety protection in case of a fault causing a high voltage event along the power supply line from the high voltage area to the SELV area.